The invention relates to a method for the photothermal inspection of a part so as to detect defects or to determine characteristics of the material from which the part is made, while eliminating the disturbing effect of variations in emissivity and in absorptivity over a surface of the part on which the inspection is carried out.
It is known to carry out the detection of planar or volume defects, of variations in materials or of thicknesses of coating layers or else to measure gradients of diffusivity or of thermal conductivity in a part by a photothermal inspection process consisting in determining the characteristics of the lateral diffusion, that is to say diffusion in a plane parallel to a surface of the part under inspection, of a thermal loading produced by local warming of the part. To do this, a warming of a zone of the surface of the part is produced and at least one detector such as an infrared detector is used to determine the rise in temperature produced by the lateral diffusion of heat imparted to the heating zone, in a detection zone situated some distance from the heating zone, on the surface of the part.
To determine the variations in the lateral diffusion in the material of the part, which are representative of the defect condition of this part or of its thermal characteristics, a scanning of the surface of the part is carried out by a means of imparting heat which can be for example a laser beam, in such a way as to displace the heating zone at the surface of the part.
The signals produced by the detector(s), during the scanning by the means of imparting heat, are representative of the rise in temperature at the surface of the part, in a detection zone whose position varies simultaneously with the position of the heating zone and which is offset by a certain distance with respect to this zone. Contactless determination of the rise in temperature in the detection zone and hence of the lateral diffusion of heat in the material and of its variations is thus carried out.
The signals produced by the detector(s) during the scanning of the part by the means of imparting heat make it possible to determine the presence of defects on or beneath the surface of the material, such as cracks or interfaces perpendicular or parallel to the surface of the part or else to determine local variations in diffusivity or in thermal conductivity at the surface or under the surface of the material.
The exploitation of the signals of the detectors by an exploitation assembly comprising, for example, a microcomputer can make it possible in particular to deliver an image of the defects situated on or under the surface of the part. The device comprising the means of imparting heat, the means of scanning the surface of the part, the detector(s) and the means of exploiting the signals of the detectors then constitutes a photothermal camera which can be used to search for or characterize defects on parts made of materials of very diverse natures.
The photothermal inspection devices known in the art use devices for imparting heat which carry out a scan of the surface of the part and a detector which is sited in such a way as to receive the thermal flux emitted by a detection zone situated some fixed distance from the heating zone and which follows the displacement of this heating zone during the scan.
The flux radiated by the surface of the material in the detection zone depends on the emissivity of the surface of the material in the detection zone and on the amount of heat imparted to the heating zone which, itself, depends on the absorptivity of the part in the heating zone.
Generally, the emissivity of the surface of the part to be inspected varies from one zone to another, that is to say varies in the course of the scanning of the part by the heat-imparting source. Likewise, the absorptivity of the material generally varies during the scanning of the part.
The measurements performed are therefore not uniquely representative of the distribution of the thermal properties of the material.
In particular, the image obtained by a photothermal camera by scanning a surface of a part is not generally representative of the defect condition of this part, owing to the disturbing effect of the variations in emissivity and in absorptivity of the part.
The purpose of the invention is therefore to propose a method for the photothermal inspection of a part consisting in carrying out the scanning of a surface of the part by a means of imparting heat which imparts heat to the part, in at least one heating zone which is displaced at the surface of the part, in sensing with at least one detector the flux radiated by the surface of the part, in at least one detection zone whose position on the surface of the part varies jointly with the position of the heating zone and in formulating, by virtue of the detector, for each of the positions of a set of successive positions of the detection zone, a signal representative of the thermal flux radiated by the measurement zone, this method making it possible to eliminate the disturbing effect of variations in emissivity or in absorptivity of the part on which the inspection is carried out.
For this purpose, the scanning of the surface of the part is carried out along a determined path, in a first displacement sense, then in a second displacement sense opposite to the first, with an identical scanning rate and, for each of the successive positions of the detection zone, a differential signal is formulated by subtracting the signals obtained during the scanning in one sense and in the other of the surface of the part and the defect condition or the characteristics of the material of the part are determined on the basis of the differential signals obtained.
To provide a better understanding of the invention, the implementation of the method of photothermal inspection according to the invention will now be described, by way of non-limiting example, while referring to the appended figures, in particular in respect of the detecting of defects such as cracks in a part.